Commit 99fead8d authored Oct 09, 2017 by Paolo Valente Committed by Jens Axboe Oct 09, 2017

block, bfq: fix unbalanced decrements of burst size

The commit "block, bfq: decrease burst size when queues in burst
exit" introduced the decrement of burst_size on the removal of a
bfq_queue from the burst list. Unfortunately, this decrement can
happen to be performed even when burst size is already equal to 0,
because of unbalanced decrements. A description follows of the cause
of these unbalanced decrements, namely a wrong assumption, and of the
way how this wrong assumption leads to unbalanced decrements.

The wrong assumption is that a bfq_queue can exit only if the process
associated with the bfq_queue has exited. This is false, because a
bfq_queue, say Q, may exit also as a consequence of a merge with
another bfq_queue. In this case, Q exits because the I/O of its
associated process has been redirected to another bfq_queue.

The decrement unbalance occurs because Q may then be re-created after
a split, and added back to the current burst list, *without*
incrementing burst_size. burst_size is not incremented because Q is
not a new bfq_queue added to the burst list, but a bfq_queue only
temporarily removed from the list, and, before the commit "bfq-sq,
bfq-mq: decrease burst size when queues in burst exit", burst_size was
not decremented when Q was removed.

This commit addresses this issue by just checking whether the exiting
bfq_queue is a merged bfq_queue, and, in that case, not decrementing
burst_size. Unfortunately, this still leaves room for unbalanced
decrements, in the following rarer case: on a split, the bfq_queue
happens to be inserted into a different burst list than that it was
removed from when merged. If this happens, the number of elements in
the new burst list becomes higher than burst_size (by one). When the
bfq_queue then exits, it is of course not in a merged state any
longer, thus burst_size is decremented, which results in an unbalanced
decrement. To handle this sporadic, unlucky case in a simple way,
this commit also checks that burst_size is larger than 0 before
decrementing it.

Finally, this commit removes an useless, extra check: the check that
the bfq_queue is sync, performed before checking whether the bfq_queue
is in the burst list. This extra check is redundant, because only sync
bfq_queues can be inserted into the burst list.

Fixes: 7cb04004 ("block, bfq: decrease burst size when queues in burst exit")
Reported-by: Philip Müller <philm@manjaro.org>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Angelo Ruocco <angeloruocco90@gmail.com>
Tested-by: Philip Müller <philm@manjaro.org>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Tested-by: Lee Tibbert <lee.tibbert@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

parent b5dc5d4d

block/bfq-iosched.c

+57 −2

Original line number	Diff line number	Diff line
		@@ -3726,8 +3726,35 @@ void bfq_put_queue(struct bfq_queue *bfqq)
		if (bfqq->ref)
		return;

		if (bfq_bfqq_sync(bfqq) && !hlist_unhashed(&bfqq->burst_list_node)) {
		if (!hlist_unhashed(&bfqq->burst_list_node)) {
		hlist_del_init(&bfqq->burst_list_node);
		/*
		* Decrement also burst size after the removal, if the
		* process associated with bfqq is exiting, and thus
		* does not contribute to the burst any longer. This
		* decrement helps filter out false positives of large
		* bursts, when some short-lived process (often due to
		* the execution of commands by some service) happens
		* to start and exit while a complex application is
		* starting, and thus spawning several processes that
		* do I/O (and that must not be treated as a large
		* burst, see comments on bfq_handle_burst).
		*
		* In particular, the decrement is performed only if:
		* 1) bfqq is not a merged queue, because, if it is,
		* then this free of bfqq is not triggered by the exit
		* of the process bfqq is associated with, but exactly
		* by the fact that bfqq has just been merged.
		* 2) burst_size is greater than 0, to handle
		* unbalanced decrements. Unbalanced decrements may
		* happen in te following case: bfqq is inserted into
		* the current burst list--without incrementing
		* bust_size--because of a split, but the current
		* burst list is not the burst list bfqq belonged to
		* (see comments on the case of a split in
		* bfq_set_request).
		*/
		if (bfqq->bic && bfqq->bfqd->burst_size > 0)
		bfqq->bfqd->burst_size--;
		}

		@@ -4460,6 +4487,34 @@ static struct bfq_queue bfq_get_bfqq_handle_split(struct bfq_data bfqd,
		else {
		bfq_clear_bfqq_in_large_burst(bfqq);
		if (bic->was_in_burst_list)
		/*
		* If bfqq was in the current
		* burst list before being
		* merged, then we have to add
		* it back. And we do not need
		* to increase burst_size, as
		* we did not decrement
		* burst_size when we removed
		* bfqq from the burst list as
		* a consequence of a merge
		* (see comments in
		* bfq_put_queue). In this
		* respect, it would be rather
		* costly to know whether the
		* current burst list is still
		* the same burst list from
		* which bfqq was removed on
		* the merge. To avoid this
		* cost, if bfqq was in a
		* burst list, then we add
		* bfqq to the current burst
		* list without any further
		* check. This can cause
		* inappropriate insertions,
		* but rarely enough to not
		* harm the detection of large
		* bursts significantly.
		*/
		hlist_add_head(&bfqq->burst_list_node,
		&bfqd->burst_list);
		}